1. Field of the Invention
The present invention relates to a method for producing potassium fluoroniobate crystals by which highly pure, large-sized potassium fluoroniobate crystals can be obtained in high yield and which is advantageous from the viewpoints of material cost and material-dissolving operation, and to potassium fluoroniobate crystals.
2. Background Art
Potassium fluoroniobate is used as raw material in the production of niobium powder. Since niobium powder has the effect of stabilizing carbon present in steels to inhibit intergranular corrosion, it is used as a steel additive, and this is the greatest use of niobium powder. Further, niobium alloys have been practically used for conductive tubes that are placed in the light-emitting parts of high-pressure sodium vapor lamps. Furthermore, niobium is employed as an additional element in superconductive materials and superalloys. Niobium powder is now being put to practical use for condensers.
A fractional crystallization process (Malinyak process) has conventionally been known as a method for obtaining potassium fluorotantalate crystals (K2TaF7) and potassium oxyfluoroniobate crystals (K2NbOF5.H2O) from a solution containing tantalum and niobium (see Zerikman, A. N., et al., xe2x80x9cNiobu to Tantaruxe2x80x9d (or Niobium and Tantalum), pp. 61-64, Nisso Tsushin-sha, Tokyo, Japan). This process is carried out in the following manner. A starting material, a solution containing tantalum and niobium, is diluted to such an extent that K2NbOF5 is not crystallized, and potassium chloride or the like is added to the dilute solution to crystallize K2TaF7. The K2TaF7 crystals are filtered off, and the filtrate is concentrated to precipitate K2NbOF5.H2O as coarse crystals. The coarse crystals are then dissolved in a 1-2 wt. % hydrofluoric acid solution to recrystallize potassium oxyfluoroniobate (K2NbOF5.H2O) from this solution. A problem with this process is that it is not easy to remove, as K2TaF7 crystals, all of the tantalum from the starting material, so that the coarse K2NbOF5.H2O crystals are to inevitably contain tantalum. It is difficult to reduce this tantalum remaining in the coarse K2NbOF5.H2O crystals even by recrystallization. This problem of tantalum contamination does not occur if a starting material containing niobium but no tantalum is used.
Another problem with the above process is that, in the above-described step of recrystallization, recrystallized from the hydrofluoric acid solution having a low concentration of 1 to 2% by weight is potassium oxyfluoroniobate (K2NbOF5.H2O) rather than potassium fluoroniobate. Such a problem never occurs when the corresponding tantalum-containing coarse crystals are subjected to the similar recrystallization; this problem is therefore peculiar to niobium. To obtain potassium fluoroniobate crystals, it is necessary to further conduct recrystallization by the use of a hydrofluoric acid solution having a concentration of as high as 12 to 15% by weight (see Zerikman, A. N., et al., xe2x80x9cNiobu to Tantaruxe2x80x9d, p. 107, Nisso Tsushin-sha, Tokyo, Japan). The reason why potassium fluoroniobate crystals are needed is that, since potassium oxyfluoroniobate contains a large amount of oxygen, it is not adequate as a starting material for the production of niobium powder.
The above-cited reference xe2x80x9cNiobu to Tantaruxe2x80x9d also describes the following: potassium fluoroniobate (K2NbF7) is more stable than potassium oxyfluoroniobate (K2NbOF5.H2O) in a hydrofluoric acid solution having a high concentration of 7% by weight or more. If this description is taken into consideration, it might be possible to produce, without effecting recrystallization, potassium fluoroniobate crystals in one step by adding a potassium-containing electrolyte to a niobium solution containing hydrofluoric acid in high concentration. However, we now found that, even if this method is adopted, potassium oxyfluoroniobates of other types, identified as K3Nb2F11O, for instance, by X-ray diffractometry are inevitably present in the resulting crystals unless the concentration of hydrofluoric acid in the niobium solution exceeds 30%. It is therefore not so easy to obtain satisfactorily highly pure potassium fluoroniobate even by this method.
Thus, crystals synthesized from a niobium solution tend to contain, in addition to potassium fluoroniobate (i.e., K2NbF7), a small or large amount of one or more potassium oxyfluoroniobates (i.e., K3Nb2F11O, KNb2O5F, KNbO2F and/or K2NbOF5.H2O etc.) regardless of whether the concentration of hydrofluoric acid in the niobium solution is high or low. Moreover, it is often observed that only potassium oxyfluoroniobate crystals precipitate.
We now found the following: if the first step of forming coarse crystals that may contain potassium oxyfluoroniobate and the second step of subjecting the coarse crystals to recrystallization using a hydrofluoric acid solution whose concentration is at least 12% by weight are effected in combination, highly pure potassium fluoroniobate crystals can be obtained, without forming unwanted by-products, by using as a starting material a niobium solution obtainable from solvent extraction, which is inexpensive. Namely, we found that it is possible to produce potassium fluoroniobate crystals in high yield by a method advantageous from the viewpoints of material cost and material-dissolving operation. We also found that it is possible to obtain sufficiently large-sized potassium fluoroniobate crystals in a large amount per operation if the temperature and cooling rate in the recrystallization step are properly controlled.
An object of the present invention is therefore to provide a method for producing potassium fluoroniobate crystals by which highly pure, large-sized potassium fluoroniobate crystals can be obtained and which is advantageous from the viewpoints of material cost and material-dissolving operation; and potassium fluoroniobate crystals.
To fulfil the above object, the present invention provides a method for producing potassium fluoroniobate crystals, comprising the first and second steps (a) and (b) of:
(a) adding a potassium-containing electrolyte to a starting material comprising niobium to precipitate potassium oxyfluoroniobate and/or fluoroniobate as coarse crystals, and separating the coarse crystals by filtration; and
(b) dissolving the coarse crystals in a recrystallization solvent that is an aqueous solution comprising 12 to 35% by weight of hydrofluoric acid and that has been heated to a temperature of 50xc2x0 C. or more, and cooling the solution to 40xc2x0 C. or lower at a cooling rate of less than 20xc2x0 C./h to precipitate potassium fluoroniobate as crystals.
Further, the present invention provides potassium fluoroniobate crystals consisting essentially of potassium fluoroniobate, containing 30% by weight or more of crystals having sizes of 0.5 mm or more as determined by sieve analysis.